One of us has a legal background for her sins and she still keeps her hand in from time to time. She says a project manager will oversee the entire project and ensure that each part of the team is meeting the brief. The PM will not necessarily be the main contractor but could be. The main contractor, on the other hand, will have principal contractual responsibility for the build and will progress it according to agreed specifications. It may sub contract particular elements of the main build contract to other trades. As main contractors don't want to be contractually responsible for problems with services by subs, there will usually be novation agreements or collateral warranties to enable the developer or property owner to enforce contractual obligations originally made between the main contractor and the subs.

If you are a developer or property owner without much experience in developing, or if you have too much going on, you may want or need to delegate the task of overseeing the project to a PM. Particularly with larger developments, it can be an asset to have someone experienced in development projects to ensure that the build remains on schedule and within budget and to liaise with all parties involved, On the other hand, if it's your only, fairly straight forward project, you may prefer to deal with the main contractor yourself if there is one. The main contractor will then deal with its team of subs and report to you. Alternatively, you could assemble your own team of trades and manage the project yourself, liaising with all parties: plumbers, electricians etc.

Hope that's not as clear as mud. :-)